Smaller, because some of it is gone.
Lossy compression throws away detail it predicts you won't notice, then stores what's left very compactly. Drag the quality down and watch where your browser's JPEG encoder cuts first: fine lines, small print and red on blue.
Each save is a new round of loss. Your browser re-saves the scene above 30 times in four ways. Saving at the same settings settles quickly; moving the picture by one pixel, resizing it or switching codec between saves keeps eating away at it.
At the same settings, save 30 measured 30.4 dB against 31.6 dB for save 1: most of the loss happens on the first save, because the encoder keeps making the same choices. Resize 90% and back did the most damage, falling to 19.8 dB, because each save sees a slightly different picture and rounds it differently.
Subtract the decoded sound from the original and what remains is exactly what the encoder removed. Your browser encodes an 8-second clip with Opus at four bitrates, decodes each one, lines it up to the sample and plays the difference.
"Kept / lost" is the signal-to-noise ratio between the original and what was removed, in dB. "Lost level" is the loudness of the removed part, against full scale. "Kept up to" is the highest band where the decoded sound is within 15 dB of the original.
At 24 kbit/s the file is 24.6 kB, 31× smaller than 16-bit WAV. What was removed sits 9.5 dB below the music while the full frequency range is kept. That number looks poor for a reason: Opus keeps the loudness of each frequency band, but rebuilds noisy sounds like the hi-hat as similar-sounding noise rather than the same waveform, and subtraction counts all of that as removed. Your ear hears the same hiss. The "removed" playback is normalised to half of full scale. The decoded sound came out 1 sample late; the page lined it up before subtracting.
Yes, but at moderate settings the quality loss is barely noticeable. At high quality settings (JPEG 85%), most people cannot see the difference.
No. Lossy compression permanently discards data. You cannot recover the lost information.
Every JPEG save causes quality degradation. For editing, use PNG or work in a non-destructive format. Only convert to JPEG for the final output.
85% quality is the sweet spot for most web images - excellent quality with significantly smaller file size than 100%.
Not at all. For photos, music, and video, modern lossy compression at good settings is virtually indistinguishable from lossless, with 5-100x smaller file sizes.